I agree with Philip. This is exactly the kind of use case that the DMC's change proposal #15, to create a mechanism for including optional headers, some defined while allowing arbitrary information, is intended to address.

There is a balance to be struck between bit flags and optional headers encoded as text values. The advantage of bit flags is size, they are very small. On the other hand they are very inflexible. Bit flags make sense when they are commonly needed and are well defined. Also, the easiest path forward when mapping SEED 2.4 miniSEED flags to the next generation is likely to simple retain the most important flags. If they do not fit those criteria, optional headers are probably better.